Floristic Diversity of a Shrubland in Northeastern Mexico Diversidad Florística De Un Área Dominada Por Arbustos En El Noreste De México

Floristic diversity of a shrubland in northeastern Mexico Diversidad florística de un área dominada por arbustos en el Noreste de México

Foroughbakhch1 R, MA Alvarado-Vázquez1, A Carrillo Parra2, JL Hernández-Piñero1, MA Guzmán Lucio1

Abstract. With the objective of ecologically characterize the Resumen. Con el objetivo de determinar las características ecoló- woody plant communities, twelve sampling sites were randomly se- gicas de comunidades vegetales maderables, doce sitios fueron alea- lected based on cartographic material and digital ortophotos from toriamente seleccionados en base a material cartográfico y ortofotos the Tamaulipan matorral located in Northeastern Mexico. In each digitales correspondientes al matorral tamaulipeco localizado en el site, the minimum sampling area was determined. Thereafter quad- noreste de México. En cada sitio, se determinó el área mínima de rats of 10 x 10 m were used in variable numbers for the evaluation muestreo sobre la base de cuadrantes de 100 m2 (10 x 10 m) donde of the tree and shrub layers, and of 1 m² for the herbaceous layer. In se evaluaron los estratos arbóreos y arbustivos. Para la evaluación del each quadrat, all individuals of any species were registered, measuring estrato herbáceo se utilizaron cuadrantes de 1 m2 (1 x 1 m). En cada its height and crown projection for the determination of density, can- cuadrante todas las plantas fueron identificadas, registrando la altura, opy cover, frequency and importance value for each species. In order proyección de la corona, y la densidad, frecuencia, abundancia y valor to evaluate the overall community a series of ecological indices were de importancia de cada una de las especies. Para poder evaluar el ín- used: richness (Margalef), diversity (Shannon-Weiner), equitativity dice de similitud de comunidades vegetales, se usaron una serie de ín- (Shannon), dominance (Simpson) and similarity (Sorensen and Jac- dices ecológicos: índice de riqueza de Malgalef, índice de diversidad card). A total of 160 taxa pertaining to 46 families were found. The de Shannon-Wiener, índice de equitatividad, índice de dominancia families with a greater number of taxa were the Poaceae, Asteraceae de Simpson e índice de similitud de Sörensen-Jaccard. Se registraron and Cactaceae with 19, 18 and 14 taxa, respectively, followed by the un total de 160 taxa perteneciendo a 46 familias. Las familias con un Fabaceae with 9 taxa, and the Euphorbiaceae and Acanthaceae with número mayor de taxa fueron Poaceae, Asteraceae y Cactaceae con 8 taxa. The rest of the families mostly presented less than 3 taxa, and 19, 18 y 14 taxa, respectivamente, seguido por Fabaceae con 9 taxa, 23 of them were represented by a single taxon. The estimation of the Euphorbiaceae y Acanthaceae con 8 taxa cada uno; el resto de las diversity according to the Index of Shannon-Weiner (H') showed familias presentaron menos de 3 taxa, y 23 de ellas presentaron solo that in the herbaceous and shrub strata the communities with greater un taxón. En relación a la diversidad florística, los estratos herbáceos H' were the same than those of greater richness. The greater values of y arbustivos mostraron mayor índice de Shannon-Weiner (H’) en H' varied between 1.23-1.56. The equitativity index showed a large comparación al estrato arbóreo, y por ende mayor riqueza en espe- variation in the tree stratum of the study communities, reaching val- cies. El valor de H’ varió entre 1,23-1,56; el índice de equitatividad ues between 0.20-0.90. mostró una mayor variación en el estrato arbóreo alcanzando valores entre 0,20-0,90. Keywords: Tamaulipan thornscrub; Diversity; Ecological index; Scrubland; Plant communities. Palabras clave: Matorral tamaulipeco; Diversidad; Índice ecoló- gico; Arbustos; Comunidad vegetal.

1 Universidad Autónoma de Nuevo León, Facultad de Ciencias Biológicas, Depto. de Botánica San Nicolás de los Garza, N.L. México. 2 Universidad Autónoma de Nuevo León, Facultad de Ciencias Forestales, A.P. 41, 67700 Linares, N.L., México. Address Correspondence to: R. Foroughbakhch, Universidad Autónoma de Nuevo León, UANL, Facultad de Ciencias Biológicas. Ave. Universidad S/N Ciudad Universitaria San Nicolás de los Garza, Nuevo León. C.P. 66451 México. e-mail: [email protected] [email protected] Recibido / Received 25.IX.2012. Aceptado / Accepted 2.XI.2012.

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INTRODUCTION MATERIALS AND METHODS

Our study was conducted in the coastal plains of the Gulf Study area. This study on the growth and structural vege- of Mexico, where vegetation consists of small trees and shrubs, tation analysis was conducted in a typical semiarid zone of the referred to as the “thorny and subinermal matorral”, “Tam- Gulf of Mexico coastal plain. This is because the vegetation of aulipan thorny scrub” or “Tamaulipan shrubland”, in most of this area is mostly inhabited by a woodland community domi- the territory (Ludwig et al., 1975; Rzedowski, 1978; Forough- nated by small trees and shrubs, commonly referred to as the bakhch, 2001). The Tamaulipan shrubland occurs extending “thorny and spiny matorral” or the “tamaulipan thorny scrub” from the coastal plain of the Gulf of Mexico to the southern (Rzedowski, 1978). The matorral extends from the Gulf coast rim of Texas State, USA. In these arid and semi-arid zones to the southern rim of Texas State, USA (Foroughbakhch, of northwestern Mexico a great variability in climatic and 2009). The species present are of shrub type with dominance edaphic conditions causes extremely diverse shrublands in of deciduous thorny elements, but thorns are lacking in some terms of the species composition, height, density and plant of them. The overall vegetation is in a transition state between associations. The various species occurring in this region can the desert microphylous scrub, the submountain matorral, the be categorized in several groups based on their ecological mesquite shrub woodland and the low thorny forest growing adaptations and forestry use. The vegetative associations are at the low altitudes, low plains and hills (Heiseke & Forough- composed of almost 60 woody species, and many of them are bakhch, 1985). important in forestry and silvopastoral activities such as the The investigation was carried out in an area of 65103 production of timber, posts, firewood, food, medicines, handi- km2 (23° 10’ 27’’ to 27° 46’ 06’’ North, 98° 26’ 24’’ to 101° 13’ crafts, and forage (Berni et al., 1979; Reid et al., 1987; Bain- 55’’ West) which included the Rio Grande river hydrologi- bridge et al., 1990; Alden, 1995). cal basin (39661 km2), San Fernando-Soto La Marina area Woody plants of dryland forests are an economically (11521.7 km2) and El Salado area (12373 km2). important forage resource in the rural population (Maydel, The climate is in extreme contrasting. The hot and dry cli- 1996). Grazing and fragmentation at a large scale have been mate predominate and, according to the Köpen classification practiced for 350 years in these areas. However, the vegetation (García, 2004), it is associated to the (1) “B” dry, (2) “Bw” arid of this region has been lately undergoing important changes or very dry, or (3) “Bs” semi-arid or semi-dry climate. Weather in structure, floristic composition, and a slow (and perhaps is very hot most of the year, mainly in the plains, but from irreversible) degradation. This has been the result of diverse November to January it is cold. The mean annual temperature human activities such as the selective plant cutting of various is 22.3 °C with a large difference between winter and sum- genus: Prosopis, Helietta, Condalia and Acacia (Ffolliott et al., mer (-2.3 to 41.1 °C). Hails and frosts usually occur every 1982; Pasiecznik et al., 2001; Estrada et al., 2004, 2005). year even after the beginning of the growing season in March. People have altered the ecological balance over a large The long-term mean annual precipitation varies from 380 mm surface area because of overexploitation, degradation and (arid zones) to 749 mm (semiarid zones), with two peaks in elimination of the woody plants from the shrublands to pro- late May and July – September, and severe drought periods in duce wood, timber and charcoal. The result of their activity June and winter. The water budget is unbalanced. The ratio of has been unfortunate for the plant resource sustainability, the precipitation to free evaporation is 0.48, and precipitation to elimination of the environmental service that this plant re- potential evaporation is 0.62 (Navar et al., 1999). source offers, and the socioeconomic profits that contribute The soils of the region are basically stony of Upper Creta- to the community dwellers. The situation becomes even worse ceous siltstone. Commonly, depth of the A horizon is about when the incidence of climate disturbances such as drought 30 cm in arid zones, and 40 cm in semiarid zones, with a pH are considered, which may carry to the desertification of the of 7.5 – 8.5 and abundant limestone. The C horizon under- zone (Burkart, 1976, Eviner, 2003; Fahring, 2003; Bucur, neath is about 80 cm deep and contains fissured limestone. In 2006; Carrillo et al., 2008). the FAO classification, these soils are classified as rendzine, Research on structure and use of matorral lands is increas- vertisoil, feozem and castañozem with low organic matter ingly important for the sustained well-being of the rural popula- content, and low levels of phosphorus and nitrogen (FAO- tion, who largely depends on the plant resource. Nevertheless, UNESCO, 1974). Underground water is hard but not saline. the literature on structural analysis and development of matorral species is insufficient to elucidate the relationships between veg- Selection of sampling sites. Sampling sites were delim- etation changes and causal factors. The objective of this study was ited based on thematic maps of land use and vegetation, scale to know the floristic diversity of the shrubby vegetation occurring 1:50000, and digital ortophotos published by the INEGI under the current natural conditions. This will contribute to the (Medina & Natividad, 1998). They were numbered in a pro- implementation of a management program for the sustainable gressive form and selected at random to determine the mini- utilization of the vegetal resource present in semiarid zones. mum size of the quadrat, according to the species-area curve

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(Mueller-Dombois & Ellenberg, 1974; Franco et al., 2001). to the botanical composition and the heterogeneity degree. Twelve of these sites were visited for their correct localization, All the tree, shrub and herbaceous species present within each and the recognition of the existent vegetal communities. On of the quadrats were identified. Measurements of plant (1) each selected site, a GPS receiver (Global Positioning System) height (m), (2) basal diameter (cm), (3) crown projection area was used to register the geographical coordinates and the el- (m2), (4) dominance (absolute and relative), (5) frequency (%) evation above the sea level (Table 1). and (6) density (absolute and relative) were obtained (Avery & Burkhart, 2002; Diéguez et al., 2003). The following equa- tions were employed to obtain some of the measured variables Table 1. Sampling sites in northeastern Mexico and their geo- (Graf & Sayagues, 2000; Mostacedo & Fredericksen, 2000): graphical coordinates. Tabla 1. Sitios de muestreo en el noreste de México y sus coordena- das geográficas. Absolute density = Number of individuals of any given species / Sampling area

Geographic coordinates Relative density = Number of individuals of any given species x 100 UTM Sampling Geographic Total number of individuals of all species sites zones Elevation East North (m) Absolute dominance of any given species = Mean basal area / 1. Plan de Great Plains of 457694 2883106 152 tree x No of trees of the study species oregano (PO) North America 2. Colorado de Great Plains of 400906 2927506 239 Relative dominance of any given species = Arriba (CA) North America Absolute dominance of the study species x 100 3. El Llano Great Plains of 437024 2854589 193 Absolute dominance of all species (EL) North America 4. La Barretosa Great Plains of 451990 2858539 166 Relative frequency of any given species = (BA) North America Frequency of the study species x 100 5. Emiliano Great Plains of 431651 2937498 164 Total frequency of all species Zapata (EZ) North America 6. Puente Río Great Plains of 413460 2982484 146 The importance value (IV) was calculated for each species ac- Salado (PS) North America cording to the following equation: 7. Loma Larga Great Plains of 448858 2904109 147 (LL) North America Importance value = Rel. Dens + Rel. Dom. +Rel. Frec 8. Los Ëbanos Great Plains of 453517 2824931 196 3 (LE) North America 9. El Álamo Great Plains of 421068 2929095 195 Diversity and Similarity between communities. The pa- (EA) North America rameters given below (i.e., richness, diversity, dominance and 10. Dulces Gulf Coastal 394236 2844953 351 similarity) were estimated as follows: Nombres (DN) Plains 11. H. San Gulf Coastal 384066 2867217 369 Richness: The index of Margalef (Dmg) was selected to Pedro (SP) Plains consider the relation between the number of occurring spe- 12. Higueras Gulf Coastal 398119 2872672 503 cies in the community or stratum (S) and the total number of (HI) Plains individuals of all species (N) (Gurevitch et al., 2002):

Dmg = S-1/ ln N

Vegetation structure analysis. The minimum area at each Diversity: The index of Shannon-Weinner (H’) was chosen site was estimated following Mueller-Dombois & Ellenberg to have a measurement of the diversity of species and its pro- (1974) and Salvador & Álvarez (2004) to obtain a representa- portional abundance in the community (Franco et al., 2001): tive botanical composition. The species occurring in each sub- sample were registered, and the minimum sampling area was H’= -∑ [pi ln (pi)] determined as the surface where at least 95% of the species of the plant community were present. Where: pi = the proportion of individuals in the ith (pi = ni/N), A 10 x 10 m quadrat was delimited at random within each n = number of individuals of the ith species, study site. The average distance among plots varied according N = number of individuals of all the species.

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Equitativity: It is a measurement of the balance in the cur between the accumulated cover from the three strata, abundance of any species in the community. In this case a and even from the interior of each stratum. The maximum measurement derived from the index of Shannon-Weiner vegetation cover was reached in the locality “Dulces Nom- (H’) was used. Calculations were made following Gurevitch bres” (Gulf Coastal Plains) with 314, 08%, where vegeta- et al. (2002): tion was very dense and uncovered soil surface area was small. On the other hand, the minimum canopy cover was E= H’/ln S (see H’ and S above for explanation) reached in “Plan de Orégano” (108.37%) on the Great Plains of North America; at this location, the plant com- Dominance: The index of Simpson (L) was used to esti- munity had very few tree individuals and large uncovered mate the probability that two individuals taken at random soil surface areas. from the community would belong to the same species: All communities located on the Great Plains of North America showed a greater similarity than those at the Gulf L = ∑ pi2 (Gurevitch et al., 2002) Coastal Plains, with values between 71% (“Plan de Orega- no”) and 49% (“Loma Larga”) (Table 2). Within the Gulf Similarity: The similarity between communities was esti- Coastal Plains, “Hacienda San Pedro” showed a similarity in mated at a general level, and for each one of the evaluated most cases lower than 40%, and “Puente del Río Salado” and strata. For their estimation, the qualitative indices of Sorensen “Higueras” in the range from 30 to 50%. and Jaccard were used as follows: Frequency. In the shrub and arboreal strata there were a Jaccard Index: Sj = a / a + b + c total of 62 taxa in the 12 localities, with an average of 21.75 Sörensen Index SSD = 2a / 2a + b + c taxa per site. Three species Celtis pallida, Opuntia leptocaulis, and Prosopis glandulosa appeared in all the study communities, Where: a = No. of species at both sites; b = No. of species at and showed the greatest values of frequency (100%). The high the second site, and c = No. of species at the first site. frequencies of Opuntia engelmanii (91.67%) and Acacia rigidula, Castela texana, Karwinskia humboldtiana and Ziziphus obtusifolia were also relevant, and all of them were present in 10 RESULTS sites (83.33%). The rest of the species exhibited smaller fre- Floristic Composition. A total diversity of 160 taxa per- quencies, and approximately a third of them were hardly pres- taining to 46 families was found. The families with greater ent in a single site (Table 3). Acacia rigidula was second in the number of taxa were Poaceae, Asteraceae and Cactaceae rank, despite occurring in 50% of the localities. The rest of the with 19, 18 and 14 taxa respectively, followed by Fabaceae species presented smaller frequencies and three species (Acacia with 9 taxa, and Euphorbiaceae and Acanthaceae with 8 taxa farnesiana, Cercidium sp. and Havardia pallens) appeared only in both cases. The rest of the families mostly presented less in one community. than 3 taxa, and 23 of them were represented by a single Prosopis glandulosa, a component of the tree stratum, taxon. The most common species present in all sampled showed quite greater frequency values than those observed for communities were Celtis pallida, Opuntia leptocaulis, Prosopis the herbaceous stratum. However, its relative frequency was glandulosa and Ruellia nudiflora.Other species, also common low in many of the sites, with values generally smaller than at the study sites, included Acacia rigidula, Acleisanthes ob- 5%, although in the “Hacienda San Pedro” it reached a relative tuse, Castela texana, Dyschoriste decumbens, Karwinskia hum- frequency of 20%. boldtiana, Opuntia engelmannii, Tridens muticus and Ziziphus obtusifolia. Importance Value (IV). Prosopis glandulosa was the species that presented the greatest importance value (IV) Density, Canopy Cover and Similarity between Com- in the tree stratum of all communities, with an average munities. The total number of species in any given commu- of 80%, followed by Havardia pallens and Acacia wrightii, nity varied from 22 to 75, with a general average of 46.91 which showed average values of 21 and 14%, respectively species per community. The vegetal stratum with the highest (Table 4). The higher importance value for all strata was diversity was the herbaceous layer, whereas the tree stratum obtained by P. glandulosa in all communities. Its IV reached displayed the lowest diversity. 100% in three of the study communities, more than 75% The average density of individuals per site was 274015.48 in six communities, and it exceeded 40% in three of the individuals/ha (range= 97900 - 466233), where 91% of them remaining communities. The species with greater IVs after corresponded to the herbaceous layer. P. glandulosa were H. pallens (21%), A. wrightii (19%) and The average cover per site was 203, 2%, taking into con- A. rigidula (18%). sideration that overlapping between individuals might oc-

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Diversity Parameters. Richness (Dmg). Within the shrub Dmg = 5.54, followed by “El Llano” (Dmg = 5.02) and “Los stratum, the community with greater richness [according to Ebanos” (Dmg = 4.86). The shrub communities with smaller the index of Margalef (Dmg)] was “Dulces Nombres” with a richness were: “Loma Larga” and “Hacienda San Pedro” (Dmg

Table 2. Similarity matrix between communities (considering the three layers) according to the indices of Jaccard and Sorensen. Tabla 2. Matriz de similaridad entre comunidades (considerando los tres estratos) de acuerdo a los índices de Jaccard y Sorensen.

Great Plains of North America Gulf Coastal Plains Site PO CA EL BA EZ PS LL LE EA DN SP HI

30 19 22 20 17 16 22 25 20 20 8 14 Shared species PO Jaccard Index % 100 33.93 36.07 33.33. 31.48 44.44 55.00 31.25 25.97 27.40 16.67 18.67 Sorensen Index % 100 50.67 53.01 50.00 47.89 61.54 70.97 47.62 41.24 43.01 28.57 31.46 45 30 27 27 16 20 33 36 26 14 20 Shared species CA Jaccard Index % 100 44.12 39.71 45.76 31.37 35.09 37.93 47.37 31.71 24.56 23.81 Sorensen Index % 100 61.22 56.84 62.79 47.46 51.95 55.00 64.29 48.15 39.44 38.46 53 34 24 16 25 38 34 26 13 24 Shared species EL Jaccard Index % 100 49.28 34.29 27.12 41.67 42.22 39.53 28.89 19.70 27.27 Sorensen Index % 100 66.02 51.06 42.67 58.82 59.38 56.67 44.83 32.91 42.86 50 25 15 24 37 36 28 15 25 Shared species BA Jaccard Index % 100 37.88 26.32 41.38 42.05 44.44 32.94 24.59 29.76 Sorensen Index % 100 54.95 41.67 58.54 59.20 61.54 49.56 39.47 45.87 41 15 16 27 30 24 14 20 Shared species EZ Jaccard Index % 100 31.25 28.07 303.4 38.46 30.00 26.42 25.00 Sorensen Index % 100 47.62 43.84 46.55 55.56 46.15 41.79 40.00 22 16 18 16 12 8 13 Shared species PS Jaccard Index % 100 42.11 22.78 21.92 16.44 20.00 19.12 Sorensen Index % 100 53.26 37.11 35.96 28.24 33.33 32.10 32 26 22 17 9 18 Shared species LL Jaccard Index % 100 32.10 28.57 21.79 18.37 24.66 Sorensen Index % 100 48.60 44.44 35.79 31.03 39.56 75 42 37 16 33 Shared species LE Jaccard Index % 100 42.00 36.63 18.82 32.67 Sorensen Index % 100 59.15 53.62 31.68 49.21 67 31 16 31 Shared species EA Jaccard Index % 100 31.31 20.78 32.63 Sorensen Index % 100 47.69 34.41 49.21 63 17 27 Shared species DN Jaccard Index % 100 23.61 28.42 Sorensen Index % 100 38.20 44.26 26 16 Shared species SP Jaccard Index % 100 23.19 Sorensen Index % 100 37.65 59 Shared species HI Jaccard Index % 100 Sorensen Index % 100 PO= Plan de oregano, CA = Colorado de Arriba, EL = El Llano, BA= La Barretosa, EZ= Emiliano Zapata, PS= Puente Río Salado, LL= Loma Larga, LE= Los ébanos, EA= El Alamo, DN= Dulces Nombres, SP= San Pedro, HI= Higueras.

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Table 3. Species with higher frequency (%) in the herbaceous, shrub and arboreal strata in the studied communities. Tabla 3. Especies con mayor frecuencia (%) en los estratos herbáceo, arbustivo y arbóreo en las comunidades estudiadas.

Herb Freq Shrub Freq Tree Freq Opuntia leptocaulis 100.00 Celtis pallida 100.00 Prosopis glandulosa 100.00 Ruellia nudiflora 100.00 Opuntia leptocaulis 100.00 Acacia rigidula 50.00 Dyschoriste decumbens 91.67 Prosopis glandulosa 100.00 Acacia wrightii 33.33 Acleisantes obtusa 83.33 Opuntia engelmannii 91.67 Celtis pallida 33.33 Tridens muticus 83.33 Acacia rigidula 83.33 Bumelia celastrina 16.67 Mammillaria heyderi 75.00 Castela texana 83.33 Condalia hookeri 16.67 Bouteolua trifida 66.67 Karwinskia humboldtiana 83.33 Porlieria angustifolia 16.67 Malvastrum coromandelianum 66.67 Ziziphus obtusifolia 83.33 Acacia farnesiana 8.33 Rivina humilis 66.67 Acacia wrightii 75.00 Cercidum sp 8.33 Setaria macrostachya 66.67 Porlieria angustifolia 75.00 Havardia pallens 8.33 Freq= Frequency. Freq = Frecuencia, Herbs= Hierbas, Shrub= arbusto, Tree = árbol.

= 2.73 in both cases), and “Puente Rio Salado” (Dmg = 2.00) Equitativity (E). The equitativity in the shrubby stratum (Table 5). reached its highest value in the communities “El Llano”, Diversity (H´). Those communities with greater H’ were “Loma Larga” and “Higueras”, with a value of E = 0.80. the same than those with greater richness in the shrub and On the other hand, the communities with minor equitativ- herbaceous strata. Within the shrub stratum, the greater ity values were “Colorados de Abajo” (E = 0.72), “Puente values of H’ were found in “El Llano” (2.73), “Los Ebanos” del Río Salado” (E = 0.71) and “La Barretosa” (E = 0.70) (2.69) and “Dulces Nombres” (2.57). The communities with (Table 5). smaller diversity were “Colorados de Abajo” (H’= 2.09) and Dominance (L). The community with greater dominance in “Hacienda San Pedro” (H’= 1.85) in the shrub stratum, and the herbaceous and shrub strata was “Puente del Río Salado” “Puente del Río Salado” (H’= 0.70) in the herbaceous stra- (Great Plains of North America) with values of L = 0.75 and tum (Table 5). L = 0.25, respectively. It was followed by “Hacienda San Pedro”

Table 4. Species with higher average importance values (IV) in the herb, shrub and arboreal strata from the studied communities. Tabla 4. Especies con mayores valores de importancia (IV) promedio en los estratos herbáceo, arbustivo y arbóreo de las comunidades estudiadas.

Herbaceous IV Shrub IV Tree IV Ruellia nudiflora 14.30 Opuntia leptocaulis 14.31 Prosopis galndulosa 80.31 Opuntia leptocaulis 8.14 Opuntia engelmanni 10.41 Havardia pallens 20.72 Ruellia sp 7.69 Prosopis glandulosa 9.87 Acacia wrightii 14.00 Calyptocarpus vialis 7.07 Celtis pallid 9.65 Acacia rigidula 11.56 Bouteloua trifida 6.41 Paulothamus spinescens 8.89 Pithecellobium flexicaule 8.51 Oxalis dichondraefolia 6.20 Acacia rigidula 7.56 Celtis pallida 7.97 Tridens muticus 5.83 Ziziphus obtusifolia 7.01 Condalia hookeri 7.39 Lantana camara 5.25 Eupatorium sp 5.76 Bumelia celastrina 6.92 Dyschoriste decumbens 5.11 Karwinskia humboldtiana 5.57 Acacia farnesiana 5.88 Setaria sp 4.92 Castela texana 5.47 Cercidium sp 3.35 For the Shrub and herbaceous strata only those species occurring in at least 25% of the sites were considered. In the tree stratum all species were considered. IV = Valor de importancia, Shrub = Arbustos, Tree = árbol.

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(Gulf Coastal Plains, L = 0.22) in the shrub stratum. The rest DISCUSSION of the communities displayed dominance values in the rank of 0.08-0.17, being “El Llano” (L = 0.10) and “Los Ebanos” (L = Comparisons between different ecosystems in semiarid 0.10) (Great Plains of North America) or “Los Ebanos” (L = zones are important to determine general trends to find out 0.08) the communities with the smallest dominance values in explanatory variables that could predict changes that may the shrub or herbaceous stratum, respectively (Table 5). occur after any alteration to the environment due to usual Table 5. Diversity values obtained in the sampling sites. Tabla 5. Valores de diversidad obtenidos en los sitios de muestreo.

Great Plains of North America Gulf Coastal Plains SITE PO CA EL BA EZ PS LL LE EA DN SP HI Herb Stratum N 932 1232 753 399 693 237 358 1340 1375 993 446 810 S 16 35 32 32 30 14 21 56 56 50 19 50 QN 31 50 30 30 25 25 25 30 30 25 30 25 D 30.06 24.64 25.10 13.30 27.72 9.48 14.32 44.67 45.83 39.72 14.87 32.40

Dmg 2.19 4.78 4.68 5.18 4.43 2.38 3.40 7.64 7.61 7.10 2.95 7.32 H' 1.68 2.59 2.11 2.59 2.22 0.70 2.36 2.93 2.82 2.79 2.08 2.86 E 0.61 0.73 0.61 0.75 0.65 0.27 0.78 0.73 0.70 0.71 0.71 0.73 L 0.27 0.11 0.25 0.11 0.16 0.75 0.12 0.08 0.13 0.10 0.16 0.10 Shrub stratum N 318 486 393 459 239 149 242 392 442 385 39 184 S 18 18 31 30 21 11 16 30 23 34 11 18 SA 700 1000 600 600 500 500 500 600 600 500 600 500 D* 4542.86 4860.00 6550.00 7650.00 4780.00 2980.00 4840.00 6533.33 7366.67 7700.00 650.00 3680.00

Dmg 2.95 2.75 5.02 4.73 3.65 2.00 2.73 4.86 3.61 5.54 2.73 3.26 H' 2.20 2.09 2.73 2.38 2.36 1.71 2.22 2.69 2.49 2.57 1.85 2.32 E 0.76 0.72 0.80 0.70 0.77 0.71 0.80 0.79 0.79 0.73 0.77 0.80 L 0.16 0.17 0.10 0.13 0.13 0.25 0.14 0.10 0.11 0.14 0.22 0.12 Tree Stratum N 10 25 10 19 12 6 16 19 34 78 32 21 S 1 4 3 2 2 1 1 3 7 8 2 4 SA 700 1000 600 600 500 500 500 600 600 500 600 500 D* 142.86 250.00 166.67 316.67 240.00 120.00 320.00 316.67 566.67 1560.00 533.33 420.00

Dmg 0.00 0.93 0.87 0.34 0.40 0.00 0.00 0.68 1.70 1.61 0.29 0.99 H' 0.00 0.82 0.64 0.66 0.29 0.00 0.00 0.54 1.56 1.51 0.14 1.23 E - 0.59 0.58 0.95 0.41 - - 0.49 0.80 0.72 0.20 0.89 L 1.00 0.56 0.66 0.53 0.85 1.00 1.00 0.72 0.26 0.29 0.94 0.33 N = number of sampled individuals; S = Number of species; QN = quadrat number (1 m²); D = Density (# individuals/m²); Dmg = Mar- galef index (Richness); H’ = Shanon Weiner´s index (Diversity); E = Equitativity of Shannon; L = Simposn’s index (Dominance); SA = Sampling area; D*= Density (# individuals/ha); D = Density (# individuals/m2). PO= Plan de oregano, CA = Colorado de Arriba, EL = El Llano, BA= La Barretosa, EZ= Emiliano Zapata, PS= Puente Río Salado, LL= Loma Larga, LE= Los ébanos, EA= El Alamo, DN= Dulces Nombres, SP= San Pedro, HI= Higueras.

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anthropogenic activities in those areas. However, these trait scrub and woodland, which affect the density and the insola- comparisons need to be made in an integrated manner be- tion degree throughout the depth of the plant layer. It might tween the different disciplines involved in the study. be observed that the presence of cetaceous species is common The number of total species by community varied enor- under higher insolation conditions, whereas the occurrence of mously from the Great Plains of North America (“Puente climbing plants associated to different strata is favored under del Río Salado”, “Los Ebanos”) to the Gulf Coastal Plains shady vegetation areas. (“Higueras” and “H. San Pedro”). The most common species, The physiognomy of the studied sites was dominated by considering those present in all the sampled communities, shrub and tree plants. The most outstanding species by its were Ruellia nudiflora, Opuntia leptocaulis, Celtis pallida and height, density and cover were H. pallens, A. wrightii, A. rigid- Prosopis glandulosa. Other species also common in the study ula, E. ebano and C. pallida. In all the studied sites, the mes- sites were Opuntia engelmannii, Dychoriste decumbens, Acacia quite populations corresponded to the species P. glandulosa. rigidula, Castela texana, Karwinskia humboldtiana, Ziziphus These results are in concordance with those of Jurado & Reid obtusifolia, Acleisanthes obtuse and Tridens muticus. (1989) and Martinez & López (2003). It is important to emphasize that we detected the presence The most common species in addition toP. glandulosa in of two endangered species during the study in compliance the tree stratum were Acacia rigidula, Acacia wrightii, Celtis with the Official Mexican NOM-059-SEMARNAT-2001 pallida, Bumelia celastrina and Condalia hookeri. (SEMARNAT, 2002). The species wereEchinocereus poselgeri In the shrub layer, the most common species were Celtis (Cactaceae), which has the status “subject to special protec- pallida, Opuntia leptocaulis, P.glandulosa, O. engelmannii, Acacia tion”, and Manfreda longiflora (Agavaceae), with “threatened rigidula, Castela texana, Karwinskia humboldtiana and Ziziphus status”. In both cases, deforestation and extensive cattle pro- obtusifolia. In the herbaceous layer, the dominant species were duction are the factors that contribute most to that negative Ruellia nudiflora, Opuntia leptocaulis, Dyschoriste decumbens, impact, and inflict loss and deterioration of plant habitats. Acleisanthes obtusa, Tridens muticus and Mammillaria heyderi. Another relevant factor for survival is the nodricismo required In general terms, the increase in the number of stems (and by the species as in the case of E. poselgeri. Likewise, the main consequent enlargement of the crown area) plays an impor- problem for the establishment of M. longiflora is the modifica- tant role in the structure and distribution of thornscrub com- tion of the habitat, and the succulent character of the plant, munities. This criterion was particularly noteworthy in the which makes it attractive to predators. Similar results were case of Prosopis spp; in most of the studied sites, it was com- obtained by Velazco-Macias et al. (2011). mon to observe its presence in disturbed areas due to cut- In the herbaceous layer, maximum and minimum density ting of trees for the supply of wooden pole fence stands and values were found in the communities at “El Alamo” and “Pu- vegetal charcoal. Growth and proliferation of secondary stems ente del Río Salado”, respectively. as a consequence of damage or injury to plants of that spe- Regarding the average cover by site, it was observed a great cies has been described elsewhere (e.g., Felker, 1996; Harris overlapping between individuals as a result of the accumu- et al., 1998). Most woody trees in thornscrub communities lated cover from the three strata, and even from the interior of are composed of only one stem each in undisturbed areas, but each layer. The maximum vegetation cover was reached in the they tend to sprout out several branches after occurrence of locality “Dulces Nombres”, community with very dense veg- any disturbance. etation and little uncovered surface area. On the other hand, the minimum canopy cover appeared in “Plan de Orégano”, CONCLUSIONS which had a vegetal community with very few trees and large, open uncovered soil surface areas. The physiognomy of the studied sites was dominated by According to the indices of Jaccard and Sörensen, the com- shrub and trees species where Prosopis glandulosa, Acacia spp., munities with greater similarity were “Plan de Oregáno” and Bumelia celastrina, Celtis pallida and Condalia hookeri were the “Loma Larga”, “El Llano” and “La Barretosa”, and “Colora- most outstanding vegetation by its height, density and cover. dos de Arriba” and “El Alamo”. The communities that showed We found 160 taxa belonging to 46 families, being the least similarity with those remaining were “Hacienda San Pe- families Poaceae [19], Asteraceae [18], Cactaceae [14], Faba- dro”, “Puente del Río Salado” and “Higueras”, with values in ceae [8] and Euphorbiaceae [8] the most important in num- the rank from 30 to 50%. ber. The total number of species by community varied from 22 The general appearance of thornscrub communities is de- (“Puente del Río Salado”) to 75 (“Los Ebanos”) with a general termined by the occurrence of small trees from 3 to 6 m in average of 46.91. The vegetal stratum with the highest diver- height present in the shrub and tree strata, which are generally sity was the herbaceous layer with 136 taxa (and an average of thorny species that vary in density and size (Bainbridge et al., 34.25 per studied site). The tree stratum displayed the smallest 1990). Humidity plays an important role in determining the diversity with 13 taxa, and a mean value of 3.17 taxa per site. exuberance degree and the species associated to the thorn- The most common species in addition toP. glandulosa in

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the arboreal stratum were Acacia rigidula, Acacia wrightii, Estrada, C.E., M.C. Yen, S.A. Delgado & Q.J. Villarreal (2004). Le- Celtis pallida, Bumelia celastrina and Condalia hookeri. gumbres del centro del estado de Nuevo León, México. Anales In the shrub layer, the most common species were Celtis del Instituto de Biología. Universidad Nacional Autónoma de pallida, Opuntia leptocaulis, P. glandulosa, O. engelmannii, Acacia México. Serie Botánica 75: 73-85. Estrada, C.A.E. (2005) Legumbres del estado de Nuevo León, rigidula, Castela texana, Karwinskia humboldtiana and Ziziphus México. In: R. Foroughbakhch, M.A. Alvarado, T. E. Torres, obtusifolia. In the herbaceous layer, the dominant species were J.S. Marroquín (Eds.). Tópicos selectos de Botánica 2. Departa- Ruellia nudiflora, Opuntia leptocaulis, Dyschoriste decumbens, mento de Botánica. Facultad de Ciencias Biológicas. Universidad Acleisanthes obtuse, Tridens muticus and Mammillaria heyderi. Autónoma de Nuevo León, pp.73-94. The average density of individuals per site considering the Eviner, V.T. (2003). Functional matrix: a conceptual framework for three strata was 274015 individuals/ha, where 268400, 413 predicting multiple plant effects on ecosystem processes. Annual and 5177 plants/ha belonged to the herbaceous, shrub and Review of Ecology, Evolution and Systematics 34: 455-485. tree layers, respectively. Fahring, L. (2003). Effects of the habitat fragmentation on biodi- The greatest importance value (IV) was shown by Ruellia versity. Annual Review of Ecology, Evolution and Systematics 34: 487–515. nudiflora and Opuntia leptocaulis in the herbaceous stratum, P. FAO-UNESCO (1974). Soil Map of the World, Vol. I. UNESCO, glandulosa, O. leptocaulis and O. engelmannii in the shrub layer, Paris. and P. glandulosa for the tree layer. Ffolliott, P.F. & J.L. Thames (1982). Manual sobre taxonomía de The communities with the greatest floral richness (Dmg Prosopis en Mexico, Perú y Chile. Universidad de Arizona. Tuc- = 7.64) and diversity (H’= 2.93) were at “Los Ebanos” in the son, Arizona, E.U.A Organización de las Naciones Unidas para herbaceous layer, “Sweet Names” (Dmg = 5.54, H’= 2.69, re- la Alimentación y Agricultura. spectively) in the shrub layer, and “the Alamo” (Dmg = 1.70, Felker, P. (1996). Commercializing mesquite, leucaena and cactus in H’= 1.76, respectively) in the tree stratum. Texas. In: J. Janick (ed.) Progress in new crops. ASHS Press. Al- The communities with greatest similarity values were lo- exandria, VA. 137 p. Foroughbakhch, R., L.A. Hauad, A.E. Céspedes, E.E. Ponce & N. cated at “Plan del Orégano”, “Loma Larga”, “El Llano”, “La González (2001) Evaluation of 15 indigenous and introduced Barretosa”, ”Colorados de Arriba” and “El Alamo”, which were species for reforestation and agroforestry in northeastern Mexico. situated in the Great Plains of North America. 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